The invention relates to an apparatus for separating fluid mixtures by membrane elements, which are spaced and arranged into a membrane element stack via a supporting frame that comprises outer and inner rings, said membrane elements having a boring for draining a permeating substance which boring has an inlet port for introduction of said mixture into said apparatus and an outlet port for retained substance as well as said permeating substance, wherein, in mixture compartments formed by said membrane elements between said inner and outer rings, said mixture to be separated flows past areas of said membrane elements, having a constant effectiveness, whereby the flow cross-sections of said mixture compartments vary from the introduction port to the exit port of said membrane stack.
An apparatus of the aforementioned kind is known from DE-PS 35 07 908. This apparatus has been developed for the purpose of reducing or avoiding the formation of cover layers on the liquid-selective surfaces of the membrane elements which reduce the effectiveness of the membrane elements.
Theoretical studies have shown that the formation of such cover layers on the liquid-selective surfaces of the membrane elements is influenced to a considerable extent by the tangential stress on the wall (Kulozik, Einflusse auf die Permeation von Wasser und gelosten Stoffen sowie auf den Deckschichtabtrag bei der Umkehrosmose, Fortschrittsberichte VDI Verlag, Dusseldor, 1986). Because the tangential stress on the wall changes with the square of the flow velocity of a liquid, means must be provided in the known apparatus that ascertain a constant velocity of the liquids to be separated when passing from the inlet port to the outlet port. In the aforementioned known apparatus this is achieved by shaping the mixture compartments differently, through which the mixture to be separated passes from the inlet to the outlet port, such that the flow cross sections, from the inlet port to the outlet port, are reduced in order to maintain a constant flow velocity of the mixture inside the apparatus according to the aforementioned requirement. This means, that the cross section of the flow channels of the mixture is reduced according to the volume reduction of the mixture by the removal of the permeating substance.
In the known apparatus the reduction of the flow cross section is achieved by varying the width of the inner and outer rings of the supporting frame that supports the membrane elements. Thereby the inside width of the mixture compartment is varied.
The apparatus of the prior art requires a costly and difficult production due to the large number of outer and inner rings of various widths, which also requires a large storage capacity for various parts, so that the apparatus may only be manufactured and provided at relatively high cost.
For example, the inner and outer rings of the supporting frame must be produced by injection molding, so that for each individual width of the inner and outer ring a separate mold must be manufactured. The cost for a respective injection mold are close to $ 20,000 so that, in order to produce the apparatus of the prior art, a large amount of money is required for the manufacture of the molds alone.
Another problem is the manufacturing process itself and the storage of the multitude of inner and outer rings of varying widths for the assembly of the known apparatus.
Also, close attention must be paid during the assembly of the apparatus of the prior art, so that only highly qualified personnel may assemble the apparatus.
It is therefore an object of the present invention to provide an apparatus for separating mixtures, which is of a simplified and less expensive design but achieves the same or increased separation quality compared to the known apparatus, which allows for reduced storage requirements as well as for an easy assembly by unskilled personnel, so that the apparatus of the present invention may be provided at lower cost.